FIG. 17 is a schematic sectional view of conventional plate wave element 101 disclosed in Patent Literature 1. Plate wave element 101 has base substrate 102 and piezoelectric body 103 that is formed on base substrate 102. Piezoelectric body 103 is formed on upper surface 102A of base substrate 102, and a part of piezoelectric body 103 is removed from upper surface 102A of base substrate 102. In the removed part, lower surface 103A of piezoelectric body 103 faces upper surface 102A of base substrate 102 across space 111 between lower surface 103A and upper surface 102A. Comb-shaped electrode 104 is formed on upper surface 103B of piezoelectric body 103 opposite to lower surface 103A. Comb-shaped electrode 104 excites a plate wave, and plate wave element 101 can form a resonator and a filter.
Conventional plate wave element 101 can excite a wave of high speed without considering a loss due to bulk radiation. However, conventional plate wave element 101 does not have a preferable frequency temperature characteristic. When the plate wave is excited by comb-shaped electrode 104 in plate wave element 101, the thickness of piezoelectric body 103 is required to be extremely small. The small thickness reduces the strength of the element, accordingly producing a crack due to a stress applied to piezoelectric body 103.